Some recent liquid crystal projectors use an LED as a light source for illuminating a liquid crystal panel.
In a liquid crystal projector, a liquid crystal panel needs to be irradiated with polarized light (specifically, S-polarized light or P-polarized light). Since light that is output from an LED is non-polarized light, if an LED is used as a light source for illuminating the liquid crystal panel, a polarization conversion is performed with regard to light that is output from the LED. Specifically, with respect to non-polarized light that is output from an LED, a polarization conversion is performed, in which one polarization component is selected from among linearly polarized light components, that are orthogonal each other, is converted to the other polarization component. If the polarization conversion efficiency is low, the light utilization efficiency will be reduced. If the polarization conversion is not performed at all, about half of the light that is output from the LED will not be used as an illumination light.
As a structure in which a polarization conversion element that comprises the first and second prisms is located opposite to the exit surface of the LED has been known as a structure in which a polarization conversion of light that is output from an LED is performed.
Each of the first prism and the second prism is a rectangular parallelepiped prism in which two rectangular prisms are adhered each other.
The first prism has a structure in which a polarized light separation film that transmits P-polarized light and reflects S-polarized light is formed on the adhesion surface of the two right-angled prisms such that light that is output from an LED enters the polarized light separation film at an incident angle of around 45 degrees. An exit surface from which P-polarized light exits is located in the propagation direction of P-polarized light that passed through the polarized light separation film.
The second prism has a structure in which a reflection film is formed on the adhesion surface of two right-angled prisms such that S-polarized light that is reflected by the polarized light separation film of the first prism enters the reflection film at an incident angle of around 45 degrees. An exit surface from which S-polarized light exits is located in the propagation direction of light that is reflected by the reflection film. A phase difference plate that converts S-polarized light into P-polarized light is located on the exit surface.
P-polarized light that exits from the first prism propagates in the same direction as P-polarized light that exits from the second prism does.
However, the structure using the foregoing polarization conversion device has the following problem.
Generally, in a projection type display device in which a display device is irradiated with light that is output from a light source and in which an image that is formed by the display device is projected through a projection lens, there is the constraint of etendue that is determined by the angle of divergence and the area of the light source. In order to effectively use the light that is output from the light source as projection light, the value of the product of the angle of divergence and the area of the light source needs to be equal to or less than the value of the product of the area of the display element and the acceptance angle (solid angle) that is determined by the f-number of the projection lens.
The area of each exit surface (first exit surface and second exit surface) of the polarization conversion device is around two times as large as the light emission area of an LED.
Thus, as the area of the exit surface increases, light that is not used for projection light increases due to the constraint of etendue. As a result, the light utilization efficiency will be reduced.
A polarized LED, in which a polarization conversion can be performed without increasing the area of the exit surface, has been proposed.
FIG. 1 shows an example of the structure of a polarized LED.
As shown in FIG. 1, the LED is composed of a laminate section in which reflection layer 101, p-type layer 102, active layer 103, and n-type layer 104 are successively laminated on sub mount 100.
Polarizer 106 is located opposite to the surface of n-type layer 104 of the LED. ¼ wavelength plate 105 is located on the LED side surface of polarizer 106 such that ¼ wavelength plate 105 faces the LED. Reflection layer 101 also functions as an electrode. The sub mount 100 side is the rear surface of the LED, whereas the n-type layer 104 side is the front surface of the LED.
Active layer 103 emits light (non-polarized light). Light that propagates from active layer 103 to the front surface side of the LED exits from the front surface of n-type layer 104. On the other hand, light that propagates from active layer 103 to the rear surface side of the LED is reflected in the direction of active layer 103 by reflection layer 101. The reflected light successively passes through p-type layer 102 and active layer 103 and then exits from the front surface of n-type layer 104.
Light that exits from the front surface of n-type layer 104 enters polarizer 106 through ¼ wavelength plate 105. With regard to light that enters polarizer 106, first polarized light (one of P-polarized light and S-polarized light) passes through polarizer 106, whereas second polarized light (the other polarized light of P-polarized light and S-polarized light) is reflected in the direction of the LED by polarizer 106.
Light reflected by polarizer 106 passes through ¼ wavelength plate 105 and then enters the front surface of n-type layer 104. With regard to light that enters the front surface of n-type layer 104, a part of the light is reflected on the front surface. Most of the light enters the LED.
Light that enters the LED successively passes through n-type layer 104, active layer 103, and p-type layer 102. Light that has passed through p-type layer 102 is reflected in the direction of active layer 103 by reflection layer 101. The reflected light successively passes through p-type layer 102 and active layer 103 and then exits from the front surface of n-type layer 104.
Second polarized light that has been reflected by polarizer 106 and that has passed through ¼ wavelength plate 105 twice in the process of which the second polarized light returns to the LED and in the process of which the second polarized light is reflected in the direction of polarizer 106 by reflection layer 101 is converted into first polarized light and then the first polarized light passes through polarizer 106.
In the foregoing polarized LED, with respect to non-polarized light that exits from the front surface of n-type layer 104, first polarized light directly passes through polarizer 106, whereas since second polarized light passes through ¼ wavelength plate 105 twice, the second polarized light is converted into the first polarized light and then it passes through polarizer 106.
Since the area of the front surface of the LED is nearly the same as that of the exit surface of polarizer 106, the polarization conversion can be carried out without increasing the area of the exit surface.
However, the material of reflection layer 101 needs to operate as an electrode of p-type layer 102 and to have high reflectance. At present, such a material has not been provided.
For example, Ag is known as a material that has high reflectance. If Ag is used as the material of reflection layer 101, reflection layer 101 that has high reflectance can be obtained. However, Ag does not satisfactorily operate as an electrode of p-type layer 102 (for example, p-GaN layer). In addition, since reflection layer 101 made of Ag does not have sufficient adhesion to p-type layer 102, it might deteriorate the reliability of the LED.
If a Ni layer or a Ti layer is formed between reflection layer 101 made of Ag and p-type layer 102, reflection layer 101 can operate as an electrode of p-type layer 102. In addition, this structure improves adhesion of reflection layer 101 to p-type layer 102. However, in this case, since the Ni layer or Ti layer absorbs light, the light extraction efficiency on the front surface of the LED will become proportionally lower.
Accordingly, a semiconductor light emitting device that improves the reflectance on the rear surface side of the LED has been proposed (refer to Patent Literature 1).
FIG. 2 schematically shows the structure of principal sections of the semiconductor light emitting device described in Patent Literature 1. In FIG. 2, the structure of the principal sections of the semiconductor light emitting device is simplified.
Referring to FIG. 2, p-type semiconductor layer 203, light emitting layer 204, and n-type semiconductor layer 205 are successively laminated. Transparent conductor layer 202, transparent layer 201, and metal layer 200 are successively laminated on the p-type semiconductor layer 203 side (rear surface) of the laminate structure.
Metal layer 200, transparent layer 201, and transparent conductor layer 202 make up the reflection film on the rear surface side of the semiconductor light emitting device.
Transparent layer 201 has a lower refractive index than p-type semiconductor layer 203 for a wavelength of light emitted from active layer 204. In addition, the film thickness of transparent layer 201 is equal to or greater than ¾ of the wavelength of light. Since transparent layer 201 is an insulation layer, a plurality of metal portions 206 that pierce transparent layer 201 are formed such that metal layer 200 conducts electricity to transparent conductor layer 202.
Metal layer 200 and metal portions 206 are made of, for example, Ag. Transparent layer 201 is made of, for example, SiO2. Transparent conductor layer 202 is an electrode layer of p-type semiconductor layer 203. Transparent conductor layer 202 is made of at least one from among ITO, GZO, ZnO, and AZO. P-type semiconductor layer 203 is made of, for example, GaN.
In the foregoing semiconductor light emitting device, light that is emitted from light active layer 204 and that propagates toward the rear surface side is reflected in the direction of light active layer 204 by the reflection film. Since the refractive index of the reflection film on the rear surface side (that mainly depends on the refractive index of transparent layer 201) is lower than that of p-type semiconductor layer 203, light that enters the reflection film at an incident angle that is greater than the critical angle is totally reflected. If the thickness of transparent layer 201 is equal to or greater than ¾ of the wave length of light, the problem, in which light that leaks to transparent layer 201 and that is not totally reflected reaches metal layer 200 and then the light is absorbed by metal layer 200, can be solved. Thus, the foregoing semiconductor light emitting device can realize high reflectance.
In addition, transparent conductor layer 202 effectively functions as an electrode layer of p-type semiconductor layer 203. In addition, transparent conductor layer 202 has sufficient adhesion to p-type semiconductor layer 203.